It has been recognized in the prior art that, in order to maintain the tiltable bodies of X-ray examination tables at acceptable heights above the floor and to avoid floor interference problems during tilting, the table body or chassis must be driven in such a way that it will translate while its angularity is being changed from the horizontal toward the vertical and vice-versa. At least one prior art proposal provides for tilting and translatory movement of the X-ray table body at different and changing rates, and more particularly with the rate of translation of the table body taking place at a relatively high speed at the beginning of the tilting operation while tilting movement occurs at a uniform rate. With this technique, the table body is quickly shifted translationally out of the way to obtain the necessary floor clearance. U.S. Pat. No. 3,131,301 to Barrett et al. discloses this mode of operation. A somewhat similar arrangement is present in U.S. Pat. No. 3,525,308 to Koopmans. An earlier U.S. Pat. No. 2,816,806 discloses drive means to shift an X-ray examination table body in translation and in rotation simultaneously at constant or linear rates. A further example of the patented prior art is shown in U.S. Pat. No. 2,872,584 to Schiring et al. Generally speaking, the prior art arrangements for driving tiltable X-ray tables have not been entirely satisfactory in that they have tended to be unduly complex and costly and have lacked desirable compactness for commercial application. Customarily, in the prior art, the primary driving means for the adjustable table body has been located entirely within the floor pedestal of the table and not in the table body or chassis. A much more compact and uncluttered construction is achieved by the present invention by having the primary table driving means located within one end portion of the table body or chassis and away from the pedestal.
A primary problem encountered with the arrangement proposed by the Barrett et al. patent arises as a result of the use of a sheave and cable drive means to achieve a variable rate of translation for the table body. The cable means will inevitably stretch during continued use, making it very difficult to maintain the correct tilt to translation ratio during operation. This difficulty is completely solved in the present invention by the employment of positively operating variable ratio drive gearing for tilting the table body at a non-linear or non-uniform rate while translational movement occurs at a constant rate. More particularly, in the invention, a pair of elliptical gears are utilized in the tilt or table angularity drive system to produce a relatively slow rate or tilt at the beginning of table body movement and to gradually increase the tilting rate to a maximum rate as the table body reaches the upright or vertical position. Not only does the positive drive of the invention avoid the use of flexible transmission elements which are subject to stretch and springiness, but in so doing provides for the essential floor clearance requirement during table tilting by an improved and more reliable mode of operation.